So far, gyms in my area flip so frequently that I haven't been able to hold more than one gym at a time when collecting my rewards. I know the reward for a single gym is 500 stardust and 10 coins.
Does this reward scale linearly with multiple gyms, or does it follow a different progression? I've also heard there might be some kind of bonus for holding a gym for more than a day, but haven't managed to do it. Is there a bonus for that (or any other kinds of bonuses)?